1. Field of the Invention
The present invention is directed to a method for spatially resolved measurement of the electrical activity of nerve cells by means of magnetic resonance and a device for the implementation of the method.
2. Description of the Prior Art
Basically three methods are known for measuring the electric activity of living nerve cells.
Electric potentials are measured with the electroencephalogram (EEG). An extremely good time resolution is hereby possible. Attempts are made to achieve a spatial resolution with this method, but it is extremely difficult to determine the location of activity sources (foci) from the measured potentials, for example, at the scalp. A series of imponderable parameters seems to make an exact localization unpromising. The possibility of introducing needles into the brain for the localization is extremely limited and is associated with considerable unpleasantness and risks.
In magnetic encephalography (MEG), the magnetic fields that are triggered by neuronal functions are measured with highly sensitive magnetic field sensors (known as SQUIDs). For example, U.S. Pat. Nos. 5,392,210, 5,136,242 and 5,417,211 disclose such methods. A location determination is only incompletely possible here and a specific, extremely complex device is required. Therefore, this method has not yet gained prevalence.
A promising approach for the determination of nerve activities lies in the application of the magnetic resonance. This method is frequently designated as fMRI (functional magnetic resonance imaging). Brain activities can be displayed in local signal boosts of images that are acquired with T2-weighted sequences and T3-weighted sequences. In this technique, however, the signal change does not directly arise from the nerve activities, but arises from an activity-produced increase of the blood oxygen content. The currently applied methods of activity measuring by means of MR nevertheless offer advantages vis-a-vis the other above mentioned methods; in particular, a relatively precise localization of the activity is possible. A disadvantage, however, is that the primary effect, namely a current flow, or the magnetic field associated therewith, is not directly determined, but a secondary effect, namely the blood oxygenation that accompanies the nerve activity, is monitored. For example, one disadvantage is that a change in blood oxygenation follows the triggering neuronal event only with a delay.
The literature Bandettini et al., "Processing Strategies for Time-Course Data Sets in Functional MRI of the Human brain", Magnetic Resonance in Medicine, 1993, volume 30, page 161-173 describes an fMRT method wherein, for improvement of the signal-to-noise ratio, a cross correlation is carried out between the stimulation function and the time curve of MR images that are acquired with a single-shot-EPI method.
The method known from German OS 195 29 639 for the time resolved and spatially resolved presentation of functional brain activities also uses a time cross correlation between a stimulation function with image information for improvement of the signal-to-noise ratio. The stimulation function is non-periodic and exhibits as few as possible secondary maximums in its auto-correlation function. Thus, periodic disturbances (for example heartbeats, respiration) can be separated from the activity signal.